576 APPENDIX. 



a smaller angle of vision, and thus add distinctness to them ; for we naturally feel 

 that our eye can see nearer objects more distinctly than distant ones, owing 

 to its having its sensibility diminished through the strata of air that intervene 

 between distant objects. We shall find, however, on considering the physical 

 conditions of vision, that a minimum must exist with regard to nearness, within 

 the boundary of which distinct vision becomes impossible, because the image of 

 the luminous point falls behind the retina. 



On examining all the other means by which we judge of the bulk of objects, 

 we shall find that we determine their relative size according to the angle of vision 

 if they are presented to us with an equal degree of distinctness, or according to the 

 distinctness where the visual angle is the same. In order to let an object appear 

 larger, we therefore only need to approximate it to the eye : by this the angle of 

 vision becomes enlarged, and the individual points of the body are removed further 

 from each other, so that we distinguish more points in the same object than was 

 previously possible ; since two points which form an angle of vision below 40 

 cannot be distinguished as separate. There is, however, a limit to this in the 

 refracting media of the eye, which amounts, on an average, to 8. Near ob- 

 jects are not seen perfectly distinct, because the rays issuing from each point 

 diverge too much to allow of their uniting in one on the retina. But it is 

 a well-known fact that the divergent rays which issue from the focus of a lens 

 become parallel after their passage through the same ; it is further known that 

 rays falling parallel upon a lens furnish an accurate image of a luminous point 

 within the focal distance of the lens. If, therefore, we place a lens between 

 our eye and the object, which we have approximated too near to the eye, in 

 such a manner that the object is placed exactly in the focus of the lens, the 

 rays proceeding from it will become parallel by passing through the lens, and, 

 as such, falling upon the eye, will be concentrated on our retina with perfect 

 distinctness. Since, then, the determination of size by the angle of vision depends, 

 where equal distinctness exists, on the nearness of the object to the eye, the body 

 in question will appear magnified to us, as we are enabled to distinguish a greater 

 number of distinct points than before. This is the theory of the Simple Microscope, 

 of the pocket lens, &c. The amount of magnifying power will depend on the 

 nearness of the object : the nearer the object, the shorter must be the focal 

 distance of the lens, by which the rays issuing from it are made parallel; or, as it 

 has been said, the shorter the focal distance of the lens, the greater its magnifying 

 power. Since the central angle on the same chord bears nearly an inverse pro- 

 portion to the radius of the circle to which it belongs, the angle of vision at a 

 distance of 4 from the eye will be twice as large as at a distance of 8, &c. ; and 

 we may obtain the apparent enlargement by dividing the focal distance of the lens at 

 the point of distinct vision by 8. The degree of magnifying power, therefore, in 

 the simple microscope, depends on the proximity of the object to the eye, as the 

 lens only serves the purpose of rendering vision possible so near to the object. 

 The impossibility of placing a lens between the object and our eye, when we have 

 arrived at a certain proximity, very soon shows us the limits of our magnifying 

 power. But we may obtain aid in another manner. It is a well-known fact in 

 physics, that a magnified image is created, under certain conditions, by objects 

 placed behind the lens. The image will correspond very exactly with the object 

 if the lens is well made, and many points will be represented in the latter which, 

 at the distance of distinct vision, would appear under a smaller angle of vision 

 than 40". This image may again be treated as an object, and be observed and 

 magnified through a simple microscope as long as there appear simple points and 

 lines capable of being resolved into two or more. This is the theory of the 

 Compound Microscope, in which we observe the object or image formed by one 

 lens (the object-glass), with another lens, (the eye-piece). These two instruments, 

 the simple and compound microscope, are the only two of scientific value. The 

 so-called solar microscope, or others constructed upon the same principles, but 

 illumined by a different light, the oxy-hydrogen microscope, are nothing more 

 than playthings a somewhat enlarged magic-lantern. The object can never 

 be so strongly magnified, nor with so much strictness and distinctness, by 

 such an instrument as by a simple microscope: the physical conditions them- 



